1. Field of the Invention
This invention relates to a tobacco filter comprising a fiber aggregate, at least one component of which is cellulose acetate fiber which is obtained by acetylating cellulose fiber from any one of a variety of sources such as pulp, cotton, jute, viscose rayon, copper-ammonia rayon and the like, while maintaining its fibrous state.
This invention relates more particularly to a tobacco filter comprising a fiber aggregate of a fibrous cellulose acetate fiber having an average degree of acetylation (percent of combined acetic acid) of 10 to 50 percent.
2. Description of the Prior Art
There have been proposed a variety of tobacco filters. One uses a tow of cellulose acetate fiber obtained by acetylating a cellulose raw material in a homogeneous liquid phase and spinning the resulting product to form the fiber. Another uses a tow of a viscose rayon fiber or a polypropylene fiber. Still another employs a non-woven sheet of a fiber such as pulp, viscose rayon, copper-ammonia rayon, polyvinyl alcohol, polyamides and polyolefins, or uses crepe paper.
Of these, there are commercially used filters prepared from a tow of a cellulose acetate fiber obtained by acetylation of cellulose in the homogeneous liquid phase and then spinning the cellulose acetate into fibers, filters from a non-woven fabric of pulp and from crepe paper.
The most popular fiber now in use is prepared in the following manner (as disclosed in, for example, Japanese Patent Publications Nos. 5749/59 and 5750/59). According to this method, the starting cellulose material is first acetylated in the homogeneous liquid phase and is subjected to partial saponification so as to have an acetylation degree (percent of combined acetic acid) of 51 to 56%. The flakes of the resulting cellulose acetate are then dissolved in acetone to form a spinning dope, which is subsequently subjected to dry spinning to form a tow of cellulose acetate. After the addition of a plasticizer, the tow is shaped into a filter in accordance with the known method in the art. (Hereinafter the cellulose acetate filter prepared by this method is referred to as the "homogeneous cellulose acetate filter").
The term "acetylation in a homogeneous liquid phase" used herein refers to an acetylation reaction of cellulose by the use of acetic anhydride which is employed nowadays on an industrial scale for the production of cellulose acetate fiber. During the initial stage, this reaction is carried out in a solid-liquid heterogeneous phase. Because the reaction product is soluble in the reaction solvent used, however, the reaction product is obtained in the homogeneous liquid phase at the end of the reaction. Hence, this term is hereby used specifically in order to distinguish from the heterogeneous system reaction as will be explained hereinafter in this specification for the illustration of the present invention wherein the starting material retains its original shape from the beginning until the end of the reaction and the reaction product is obtained in solid form.
The above mentioned production method for producing the homogeneous cellulose acetate filter requires an extremely long production time from the start until the final product is obtained. For the production of a filter having an especially high filtering efficiency, the preparation of fiber having a fine denier requires a high level of technical skill, thereby resulting in an increased overall cost of production.
In comparison with the above-mentioned homogeneous cellulose acetate filter, the filter from a pulp fiber sheet (such as disclosed in Japanese Pat. Publications No. 2039/70 and 10599/70) and the filter from crepe paper (such as disclosed in Japanese Patent Publication No. 5206/59) are inferior because they deteriorate the flavor and taste of the tobacco in addition to various other defects such as poor roundness of the filter, inferior shape of the cut mouthpiece, deformation of the mouthpiece due to moisture absorption during smoking and so forth.